This invention relates generally to phased array antennas and, more particularly, to phased array antenna systems that must provide multiple beams simultaneously. By adjusting the phase angles of signals received from or transmitted to multiple antenna elements in an antenna array, an antenna control system effectively steers the antenna beam, whether in a receive mode or a transmit mode.
High gain antennas are widely useful for communication purposes such as television earth station terminals for satellite communications and other sensing/transmitting uses including radar. In general, high antenna gain is associated with high directivity, which in turn arises from a large radiating aperture.
High gain antenna systems are often used in connection with television receive-only (TVRO) systems such as those in the circularly polarized direct broadcast (DBS) band and in other linearly polarized systems. TVRO systems have been available since the early 1980s to those desiring to watch television via satellite-delivered signals in their homes. A common method for achieving a large radiating aperture in TVRO applications is by the use of parabolic reflectors fed by a feed arrangement located at the focal point or focus of the parabolic reflector. Typically, large mesh or solid parabola-type antennas (i.e. backyard dishes) are placed in the yard of the consumer. Such parabolic dishes are often motorized to enable rotational movement along particular spatial arcs in which satellites are disposed thereby allowing the homeowner or consumer to view any one of a number of different satellites, one at a time. Unfortunately, movement of such parabolic antennas via the motor from one satellite to another is time consuming. For example, it may take up to two minutes or more in some instances for the motor to move a typical parabola dish-type antenna from one extreme of the arc of satellites to the other. Furthermore, mechanical systems are subject to mechanical breakdown.
Motorized parabolic antenna systems also tend to be bulky; noisy, and subject to high maintenance requirements due to their abundance of moving parts. As stated above, most such parabolic antennas can only receive one satellite signal at a time. This is because typically a parabolic antenna reflects and concentrates the received signal to its focal point. A feed is mounted at the focal point to receive the signal and direct it to an amplifier/down converter, which then directs the signal to the receiver in the home. Thus, depending upon what direction the dish is oriented; one satellite signal is focused into the focal point feed at a time.
Some prior art parabolic antennas have included multiple feeds near the center of the dish to enable the homeowner to receive multiple satellite signals simultaneously. Unfortunately, the angular range of such multi-feed systems is limited. Such multi-feed antennas typically experience a signal loss because the multi-feeds are not directly in the center (i.e. focal center) of the dish but are only in its general proximity. Additionally, parabolic antennas often include structure required to support the feed. This often adversely affects the illumination of the aperture and thereby perturbs the far-field radiation pattern.
Modern antenna systems have found increasing use of antenna arrays for high gain purposes. Phased array antennas typically consist of a plurality of stationary antenna elements that are fed coherently and use variable phase or time-delay control at each element to scan a beam to given angles in space. The primary reason for the use of such phased array antennas is to produce a directive beam that can be repositioned (scanned) electronically as opposed to by mechanical means. Variable amplitude control is sometimes also provided for pattern shaping. Single beam phased arrays are sometimes used in place of fixed aperture parabolic antennas, because the multiplicity of antenna elements allows more precise control of the radiation pattern thus resulting in lower side lobes and precise pattern shaping. Unfortunately, such systems are highly expensive and are generally for this reason not used in TVRO applications.
While phased arrays often have a single output port (per element), multiple beam antenna systems have a multiplicity of output ports, each corresponding to a beam at a different angle in space. Typical systems utilizing such multiple beam technology (and needing simultaneous, independent beams) include multiple-access satellite systems and a variety of ground-based height-finding radars. Generally, multiple beam array antenna systems utilize a switching network that selects a single beam or a group of beams as required for specific applications via a generic lens or reflector. In other cases, multiple beam arrays are used as one component of scanning systems such as the use of a multiple beam array feed for a reflector or lens system.
Array antennas generally include an array of ordinarily identical antenna elements (or plurality of elements or of sub-arrays of elements), each of which has a lower gain than the gain of the array. The antennas (or elements) are arrayed together and fed with an amplitude and phase distribution, which establishes the far-field radiation pattern. Since the phase and power applied to each element of the array can be individually controlled, the direction of the beam (transmitting and receiving) can be changed accordingly. In multiple beam systems, reflectors or lenses are used to control the beam. A salient advantage of array antennas is clearly the ability to scan the beam or beams electronically without moving the mass of a reflector as is required in prior art parabolic-type antennas.
These technologies are well tried, but they are complex and onerous in a way that rapidly increases with the size desired for the antenna. It follows from this that electronic scanning cannot be used in applications where it would be worthwhile.
For example, in conventional phased array antenna systems, each radiating element in the array has to have an independent radio frequency (RF) phase shifting circuit for each independent beam to be produced. In an illustrative satellite antenna system, an array has 547 elements and there is a requirement to produce sixteen independent beams. Thus, 8,752 phase shifting circuits are needed, together with sixteen 547-way RF power combiners to produce the sixteen independent beams. Each phase shifting circuit has to be connected to an appropriate one of the power combiners, creating a maze of crossing lines. Moreover, each of the phase shifting circuits requires its own four-bit control line to provide the requisite beam steering accuracy. The complexity of implementation increases even further as the number of independent beams rises above a modest value.
Apart from conventional phase shifters, other means have been envisaged for electronically steering multiple beams with a single aperture. This capability can be achieved by using certain beam forming networks. There are two major types of beam forming networks, namely, matrices and lenses.
One example of a matrix network is the Butler matrix described in U.S. Pat. No. 4,316,192. The Butler matrix is a linear, bilateral device with the properties of superposition and reciprocity. It has 2N input ports and 2N output ports. Typically, each output port is connected to a corresponding element of a linear array of radiating elements. Driving only one input port with a source of electromagnetic energy produces a single beam that has a direction corresponding to the input port that was selected. Driving multiple ports produces multiple beams. Each has a direction corresponding to the input port that was driven. If all of the input ports are driven, a cluster of 2N beams results. The cluster of beams may be scanned in space if beam steering elements, such as phase shifters or time delay networks, are placed between every output port of the matrix and its corresponding radiating element. However, the beam steering elements do not permit any single beam to be steered independently of any other beam.
In the Butler matrix, signal parameters such as center frequency, total bandwidth and modulation can differ from one input port to another. Thus, different signals can be launched in different directions as long as the beams are orthogonal. Furthermore, when the Butler matrix is operated in a receive-only mode, the port from which energy emerges identifies the direction from which the energy was received.
Switching beam directions is accomplished by switching input ports. When the number of simultaneous beams is large and/or when a high power level is being used (as is common in radar, communications, and electronic warfare systems) switching input ports can become complicated.
Lens beam forming networks share the same basic properties of matrices, namely, they are linear, bilateral devices with the properties of superposition and reciprocity. Lens beam forming networks operate similarly to an optical lens, i.e. the microwave lens converts a point source of electromagnetic energy into a linear phase front.
The Ruze lens, as described by Fay in U.S. Pat. No. 5,128,687, is an example of a lens beam forming network. The Ruze lens is a line source antenna that can provide multiple, independently steerable, simultaneous beams. Like other microwave lenses, it has a focal arc with each position along that arc corresponding to a different beam direction. Pointing the beam in a particular direction is accomplished by merely placing a beam launching device at the corresponding location on the focal arc of the lens. Scanning of the beam is accomplished by moving the beam launcher along the focal arc. Using multiple beam launchers produces multiple simultaneous beams, each of which may be steered independently of the other beams. In addition, the aperture of the lens can be large enough to produce the desired far field beamwidth independent of the number of resolvable beam directions that are used.
One type of beam launcher is a waveguide. Each independent beam requires its own length of waveguide. Changing the direction of any of the multiple simultaneous beams produced by the waveguide beam launchers requires the mechanical relocation of the waveguide.
An alternative to the waveguide beam launcher is an array of monopole elements, hereinafter referred to as probes, or radiating elements mounted along the focal arc. Each probe location corresponds to a specific beam direction. When driven by an electromagnetic energy source, a probe will radiate energy in a well-defined and predetermined direction. In addition, since the lens is a reciprocal device, energy received from that direction will come to a focus at that probe.
Beam pointing angles corresponding to locations between two adjacent probes can be achieved by splitting the power from the electromagnetic source between the two adjacent probes, and by amplitude and/or phase weighting of the distributed power.
Typically, a complex network of switches directing signals to the probes on the focal arc is used to achieve rapid and random-access steering of beams. The switch network is nominally the same kind of switch network that would be required to switch between input ports of a Butler matrix or any other matrix beam forming network. As with the matrix beam forming networks, in many applications the switching network must be capable of handling high power levels.
The Ruze lens is only one of many lens antennas wherein the beam direction corresponds to a location on the focal arc. Other examples of lenses include, but are not limited to, the Rotman lens as described by Sievenpiper in U.S. Pat. No. 6,982,676 and other lenses such Archer lenses described by Archer in U.S. Pat. No. 5,099,253 and U.S. Pat. No. 4,845,507.
While beam forming networks are capable of overcoming some of the practical issues that have prevented broad application of conventional phase-shifter-based electrically steered technology, improvements are still needed. Accordingly, it will be appreciated that there is a need for a less complex technique to provide multiple independent beams from a phased array antenna system. The present invention is directed to this end.
It is apparent from the above that there exists a need in the art for a multiple beam array antenna system which is small in size, cost effective, and modular to increase gain without significantly increasing cost. There also exists a need for such a multiple beam array antenna system having the potential to receive signals simultaneously from more than a single location. It is the purpose of this invention to fulfill the above-described needs in the art, as well as other needs apparent to the skilled artisan from the following detailed description of this invention.
Those skilled in the art will appreciate the fact that array antennas are reciprocal transducers which exhibit similar properties in both transmission and reception modes. For example, the antenna patterns for both transmission and reception are identical and exhibit approximately the same gain. For convenience of explanation, descriptions are often made in terms of either transmission or reception of signals, with the other operation being understood. Thus, it is to be understood that the array antennas of the different embodiments of this invention to be described below may pertain to either a transmission or a reception mode of operation. Those of skill in the art will also appreciate the fact that the frequencies received/transmitted may be varied up or down in accordance with the intended application of the system.